The only issue I have with Powerlogix's upgrade is that they replace the larger "heat-pipe" heatsink that comes stock with the Pismo with a small crappy little copper one. The Daystar G4 upgraded, however, includes an improved "heat-pipe" heatsink and heat sink cage. The Daystar G4 upgrade goes the extra mile here, it is clearly a well engineered upgrade.
But performance wise, I just don't know if a 550mhz G4 utilizing an external 1Mb L2 cache running @ 200mhz can compete with a 1ghz G3 with 1mb of 1:1 on-chip L2 Cache (1ghz). You may not notice a difference overclocking your L2 cache 50mhz, but the performance benefits of a large L2 cache running at 1ghz should help dramatically with the bottleneck of the 100mhz FSB in the Pismo's. With VirtualPC, for instance, translation of CISC to RISC on-the-fly is heavily dependent on L2 cache speed & size. These sought of situations, Altivec cannot make up for. That's what good about the G3, everything benefits from its raw speed and large fast L2 cache.
It's a shame Daystar aren't looking into 750GX upgrades for Lombards & Pismo's, they'd do a much better job than Powerlogix at it